猫鼬一对多关系未显示预期结果

Question

我是关系数据库的新手。我正在使用节点js，并表示后端，REST API和数据库是MongoDB。我正在使用猫鼬进行连接和模型。我成功连接数据库，并能够发布和获取数据。通过测试应用程序，我正在使用Postman。我创建了两个模型，一个是学生，另一个是课程。 我的目标是一名学生可以参加多门课程，并希望避免重复使用学生姓名，电话和电子邮件。 我可以发布来自Mongodb的带有Studentid的课程。当测试课程获得请求时，我可以看到课程与学生image之间的关系，但是我尝试学生获得请求时，我看不到学生与课程image之间的关系。另外，当我尝试单人学生获得请求时，我看不到这种关系image

这是我的学生模型

const mongoose = require("mongoose");
const { Schema } = mongoose;

const studentModel = new Schema({
  _id: mongoose.Schema.Types.ObjectId,
  name: {
    type: String,
    required: true,
    unique: true
  },
  birthday: {
    type: Date,
    required: true
  },
  address: {
    type: String,
    required: true
  },
  zipcode: {
    type: Number,
    required: true
  },
  city: {
    type: String,
    required: true
  },
  phone: {
    type: Number,
    required: true,
    unique: true
  },

  email: {
    type: String,
    required: true,
    unique: true
  },
  course: {
    type: mongoose.Schema.Types.ObjectId,
    ref: "course"
  }
});

const student = mongoose.model("student", studentModel);
module.exports = student;

这是我的课程模型

const mongoose = require("mongoose");
const { Schema } = mongoose;

const courseModel = new Schema({
  _id: mongoose.Schema.Types.ObjectId,
  name: {
    type: String,
    required: true
  },
  startdate: {
    type: Date,
    required: true
  },
  enddate: {
    type: Date,
    required: true
  },

  student: {
    type: mongoose.Schema.Types.ObjectId,
    ref: "student"
  }
});

const course = mongoose.model("course", courseModel);
module.exports = course;

这是我的服务器，获取并发布请求。

require("dotenv").config();
const express = require("express");
const app = express();
const morgan = require("morgan");
const helmet = require("helmet");
const cors = require("cors");
const mongoose = require("mongoose");
const student = require("./models/student");
const course = require("./models/course");
//app middlewear
app.use(morgan("common"));
app.use(helmet());
app.use(cors());
app.use(express.json()); //body Parser

//Connect to db
mongoose
  .connect(process.env.MONGODB_URI, {
    useUnifiedTopology: true,
    useNewUrlParser: true
  })
  .then(() => console.log("DB Connected!"))
  .catch(err => {
    console.log(err);
  });

//student

app.get("/students", async (req, res, next) => { //This is all students get request
  try {
    await student
      .find()
      .populate("course")
      .select("name birthday address zipcode city phone email course")
      .then(docs => {
        const response = {
          count: docs.length,
          students: docs
        };
        res.json(response);
      });
  } catch (error) {
    console.log(error);
  }
});

app.get("/students/:id", async (req, res, next) => { //THIS SINGLE STUDENTS GET REQUEST
  const id = req.params.id;
  try {
    student.findById(id).then(data => {
      console.log(data);
      res.json(data);
    });
  } catch (error) {
    console.log(error);
  }
});



app.post("/students", async (req, res, next) => {
  const logs = new student({
    _id: mongoose.Types.ObjectId(),
    name: req.body.name,
    birthday: req.body.birthday,
    address: req.body.address,
    zipcode: req.body.zipcode,
    city: req.body.city,
    phone: req.body.phone,
    email: req.body.email,

  });
  logs
    .save()
    .then(data => {
      console.log(data);
      res.json(data);
    })
    .catch(err => {
      console.log(err);
    });
});
//course
app.get("/courses", async (req, res, next) => {//THIS IS COURSE GET REQUEST WHERE I CAN SEE THE POST AND RELATIONSHIP BETWEEN COURSE AND STUDENT
  try {
    await course
      .find()
      .select("name  student startdate enddate ")
      .populate("student") 
      .then(docs => {
        const response = {
          count: docs.length,
          courses: docs
        };
        res.json(response);
      });
  } catch (error) {
    console.log(error);
  }
});



app.post("/courses", async (req, res, next) => { //THIS IS COURSE POST REQUEST
  const logs = new course({
    _id: mongoose.Types.ObjectId(),
    name: req.body.name,
    startdate: req.body.startdate,
    enddate: req.body.enddate,
    student: req.body.studentid
  });
  logs
    .save()
    .then(data => {
      console.log(data);
      res.json(data);
    })
    .catch(err => {
      console.log(err);
    });
});

const port = process.env.PORT || 5000;
app.listen(port, () => console.log(`? App is listening at port ${port}!`));